Spectral Efficiency Analysis in Cell-Free Massive MIMO Systems with Zero-Forcing Detector
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This paper investigates the spectral efficiency performance of the cell-free massive multiple-input multiple-output (MIMO) systems employing linear zero-forcing detector. We firstly derive the approximate upper/lower-bound of the achievable uplink rate for the functions of the number of base-station antennas L and the large-scale fading, with the perfect channel state information (CSI) and imperfect CSI, respectively. Those bound results for both cases are not only in the simple unified expressions, but also can degrade into the bound results in the conventional massive MIMO systems. In addition, our obtained bounds in imperfect CSI case degrade into the perfect CSI case when the pilot sequence power becomes infinite. Particularly, the asymptotic analysis shows that the obtained bounds have an asymptotic lower-bound α/2_2 L for all cases in cell-free massive MIMO thanks to the extra distance diversity offered by massively distributed antennas, which the path-loss factor α>2 except the free space environment. In other words, cell-free massive MIMO has huge potential of spectral efficiency than conventional massive MIMO with the asymptotically tight bound _2 L. Finally, the above mentioned results are verified via the Monte-Carlo simulations.
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